Internal combustion engines have many components that can affect the reliable and efficient operation of the engine. Engine operation and performance may be especially affected by the condition of those components that are associated with the engine's combustion cylinders such as intake and exhaust valves, piston rings, head gaskets and the like. Failures can occur for various reasons, such as thermal cycling, fatigue and the like. When such components fail, or their performance is compromised by a less than complete failure, the effects of such failure may not be immediately apparent to the engine's operator. However, such failures may cause a reduction in engine power, loss of sufficient sealing of the engine's combustion cylinder, increased oil consumption, decreased fuel economy, and other effects.
Even in the absence of a component-related condition, in-cylinder engine combustion may be further affected by various environmental factors such as ambient air temperature, barometric pressure, fuel quality, engine core temperature, and other factors. Such environmental factors, in addition to or instead of engine component conditions, may result in issues with engine combustion including misfire, detonation of the fuel/air mixture, and/or pre-ignition. Apart from adversely affecting engine fuel consumption, noise, roughness, emissions, and power output, improper combustion can also result in premature engine component failure, engine starting issues, and others.
Modern engines may further include variable valve timing systems, which can actively and selectively control engine valve timing. The calibration of such systems and their performance degradation over time may also affect ignition timing and cause varying degrees of abnormal engine combustion, which can in turn affect engine performance and emissions. The detection and diagnosis of abnormal engine combustion is a time consuming task because it traditionally entails running the engine in a diagnostic or service mode with instrumentation added to the engine to detect abnormalities. Moreover, abnormal combustion that is imperceptible to the user may go undetected. In the past, various attempts have been made to diagnose such engine conditions during normal engine operation by use of pressure sensors configured to measure cylinder pressure within the cylinder.
For improving the accuracy of such pressure sensors, it has been proposed to reference the cylinder pressure measurement based on a variable polytropic coefficient, as described in SAE Paper No. 2007-01-3535, by Lee et al. (“Lee”), which is entitled “An In-Cylinder Pressure Referencing Method Based on a Variable Polytropic Coefficient.” Lee describes that the successful monitoring of the combustion process depends on the accurate measurement of in-cylinder pressure. According to Lee, piezoelectric transducers are normally used for in-cylinder pressure measurement, but rapid changes in the temperature of the transducer housing and the quartz sensing element included therein can change the transducer offset voltage. Lee proposes that piezoelectric transducers require referencing the output of the absolute pressure (pegging), and the study of Lee reviews several pegging methods and proposes a modified pegging method based on a variable polytropic coefficient. However, Lee's methods are insufficient to accurately detect an in-range fault for a pressure sensor consistently and with good repeatability.